I build amplifiers at times for fun but I always have a problem with them, I get small hums and a higher pitch sound from tweeters, at times I realize when I touch like pots the humming gets higher and if I touch like a speaker wire the humming goes down or stops, I normally wonder if its grounding issues as I don't think I know how to ground my circuits when I build them. IS there something I am missing or is it that the amp and preamp circuits just isnt clean enough to stay silent.
I built a couple tda7293 basic amplifier circuit that I got from the datasheet of the chip and I normally add a tl072 preamp to it but it hums.
In audio amplifiers , hum and stray signals can be coupled into the desired signal from various sources. The most vulnerable are DC power lines and sources, front-end circuits and also magnetic field coupled from close AC transformers and power lines. Some glass-envelope diodes sense light from fluorescent tubes, etc.
It is a good idea to power the low-level circuits from a battery (the DC lines do need capacitors to block noise, too), before you connect these circuits to the main DC power lines. Then you can determine the hum/noise source, and use shields or blocking filters to get rid of them.
Modern operational amplifiers are quite wideband and can oscillate high above audio spectrum,, as well as receive AM signals from the air. Be ready to a hard work.
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Also, as you mention the pickup from touching small metal components, the new amplifier technology uses high-impedance circuits that can amplify such small voltages easier than the low-impedance circuits used earlier.
The front-end ciruits like electret microphone amplifiers require a complete grounded enclosure to avoid such pickup, in the following circuit I would recommend to use lower-impedance circuits and a good grounding (one-point is preferred).